Cardiac arrhythmias are a significant health problem, and atrial fibrillation is a common cardiac arrhythmia that may increase risk factors for other conditions such as embolisms and contribute to the onset of ventricular arrhythmia. It is believed that atrial fibrillation is caused by certain electrical signals within the heart. It is believed that cardiac electrical impulses start in a sinoatrial (SA) node, spread through the atria, and progress through the atrial-ventricular (AV) node to the ventricles to complete a heartbeat. Atrial fibrillation is an irregular heart rhythm that is believed to originate in the atria or the upper two chambers of the heart. The pulmonary veins, in particular, can be sources of disruptive re-entrant electrical impulses that cause atrial fibrillation.
One known method of treating atrial fibrillation is by use of medication that is intended to maintain a normal sinus rate and/or decrease ventricular response rates. It is also known to use implant devices such as atrial defibrillators to treat these conditions. Other known methods and devices have been developed for creating therapeutic lesions, e.g., by minimally-invasive surgical methods, in the myocardial tissue to block unwanted electrical impulses believed to be the source of atrial fibrillation. In this context, ablation has come to mean deactivation or removal of function rather than actual tissue removal.
Formation of lesions may be performed using both endocardial and epicardial devices and procedures. Endocardial procedures are performed from within the heart. Since the endocardium primarily controls myocardial functions, there are inherent advantages to generating lesions by applying ablative energy to endocardial surfaces. For this purpose, it is known to use radio frequency (RF) devices or catheters and cryogenic balloon devices. Examples of known lesion formation devices, including cryogenic balloon catheters, for use in endocardial ablation and their operation are described in U.S. Patent Application Publication No. 20060084962, U.S. Pat. Nos. 6,027,499; 6,468,297; 7,025,762; 7,081,112 and 7,150,745 and Williams, et al, “Alternative Energy Sources for Surgical Atrial Ablation”, J. Card. Surgery, 2004; 19:201-206, the contents of which are incorporated herein by reference as though set forth in full.
During use of a cryo-ablation balloon catheter, a coolant or refrigerant such as nitrous oxide is delivered to a cryogenic balloon, and cryogenic cooling results from a pressure drop as the cryogenic fluid is sprayed into the interior of the balloon, thereby causing the balloon to expand against the target tissue, which is cryogenically ablated as a result of the reduced temperature. The effectiveness of cryogenic balloon catheters depends on various factors including, for example, the accurate positioning of a cryogenic ablation device, sealing of entrances into the pulmonary veins, the ability to select the depth of lesions that are formed, and the ability to monitor or determine the effectiveness of ablation. Known devices, however, have a number of shortcomings and can be improved.
For example, known ablation devices may be therapeutic in that they treat or ablate tissue, but they are not suitable for performing diagnostics, e.g., determining locations of tissue to be ablated and determining the effectiveness of tissue ablation. Consequently, during an ablation procedure, an ablation device used for delivering a refrigerant and ablating tissue must be removed and replaced with a diagnostic device to assess the ablation, followed by removal of the diagnostic device and re-insertion of the ablation device to continue ablation as necessary. This switching of therapeutic and diagnostic devices is repeated as necessary until the desired ablation effect is achieved, but this procedure is not convenient and is time consuming.
Further, the entrance to a pulmonary vein is typically not a radially symmetrical cone and instead is typically a bent flattened cone, with an adjoining ridge of tissue. Consequently, two pulmonary veins may enter the atrium so close together that their mutual entrance forms an oval antrum. With certain known devices, when a balloon is inflated, the balloon may pop out or fail to seal an antrum entrance. As a result, blood may flow under the edge of the balloon to reduce cryogenic cooling and the ability to ablate adjoining tissue, thus preventing formation of complete circumferential lesions, which are desired to electrically isolate pulmonary veins from the atrium.